1. Field of the Invention
This invention relates generally to motor speed control circuits and, more particularly, is directed to improvements in motor speed control circuits of the type having a phase-locked loop including a frequency-divider with a changeable dividing ratio for selecting the rotational speed of the controlled motor.
2. Description of the Prior Art
It is known to provide a motor speed control circuit with a phase-locked loop (PLL) for the turntable drive motor of a phonograph record player, and in which a frequency-divider included in the phase-locked loop has its dividing ratio changed for altering the rotational speed of the controlled motor, for example, to selectively establish a turntable speed of 45 rpm or 331/3 rpm. However, in the existing motor speed control circuits of the described type, the loop gain of the phase-locked loop varies in accordance with changes in the selected rotational speed of the controlled motor. Such variation of the loop gain results in changes in the natural oscillation frequency and damping factor of the motor speed control system so that the response speed and transient response characteristic are not uniform for the various possible rotational speeds of the controlled motor. By reason of the foregoing, in a phonograph record player with the described motor speed control circuit, there is a difference between the fidelity of reproduction when the turntable is rotated at 45 rpm and at 331/3 rpm.